1. Field of the Invention
The present invention relates to a process and apparatus for removing unpolymerized monomers from solid olefin polymers. Particularly, the invention relates to a process for removing unpolymerized hydrocarbon monomers from a granular polymer of ethylene and/or propylene and one or more C.sub.4 to C.sub.8 alpha olefins. More particularly, the invention relates to a process for the removal of unpolymerized hydrocarbon monomers from "sticky polymers" such as ethylene propylene diene terpolymers.
2. Description of the Prior Art
It has long been known that olefins such as ethylene can be polymerized by contacting them under polymerization conditions with a catalyst comprising a transition metal compound, e.g., titanium tetrachloride and a cocatalyst or activator, e.g., an organometallic compound such a triethyl aluminum. Catalysts of this type are generally referred to as Ziegler catalysts.
The resulting granular polymers produced from these processes usually contain residual gaseous or liquid unpolymerized monomers including hydrocarbon monomers. These monomers should be removed from the granular resin for safety reasons, since there is a danger of explosion if the hydrocarbon monomer concentration becomes excessive in the presence of oxygen. In addition, proper disposal of the hydrocarbon is required in order to meet environmental standards concerning hydrocarbon emissions.
The prior art teaches techniques for removing volatile unpolymerized monomers from polymers of the corresponding monomers. See, for example, U.S. Pat. Nos. 4,197,399; 3,594,356, and 3,450,183, in which a columnar (or straight cylindrical) vessel is used as a purger.
U.S. Pat. No. 4,372,758, discloses a degassing or purging process for removing unpolymerized gaseous monomers from solid olefin polymers. The purging process generally comprises conveying the solid polymer (e.g., in granular form) to a column-shaped purge vessel and contacting the polymer in the purge vessel with a countercurrent inert gas purge stream to strip away the monomer gases which are evolved from the polymer. Purging efficiency of this countercurrent plug flow purger normally increases with the increase of the superficial velocity of the purge gas. However, if the gas velocity exceeds the minimum bubbling velocity (U.sub.mb) of the granular resin to be purged, the purger becomes a bubbling fluidized bed and there occur not only backmixing of purged and unpurged granular resin but also bypassing of purge gas, as bubbles, through the bed without having contact with the granular resin. These normally result in a substantially reduced purging efficiency for continuous mode operation of the purger. To avoid bubbling flow in the purger, therefore, the columnar purger is normally operated at a relatively low superficial gas velocity, below the minimum bubbling velocity of the resin. When the purger is operated at such low superficial gas velocities, however, the process tends to have uneven gas distribution in the purger. Purge gas seems to bypass through certain channels without contact with the majority of solids. This also results in a poor purging performance. This poor solid-gas contact phenomena in the conventional packed bed process may be one reason why actual purging is at least one order of magnitude worse than theoretical predictions.
Further, when producing certain types of ethylene polymers, such as ethylene propylene diene terpolymers and "sticky polymers", monomers such as ethylidenenorbornene (ENB) remaining in the product must be substantially purged from the product due to cost and environmental considerations. However ENB has a significantly low diffusivity as compared to other monomers. If a conventional columnal packed bed process is used, it would require an impractically long residence time or an extra large amount of purge gas. It is clear that such a process is not entirely suitable for ENB purging.
U.S. Pat. No. 5,292,863, ameliorates this problem by providing a process for removing unpolymerized gaseous monomers from a solid olefin polymer by utilizing a column-shaped purge vessel provided with a gas permeable, solids impermeable constraint means. An inert feed gas is fed to the purge vessel and in countercurrent contact with the polymer, the inert purge gas being utilized in an amount and at a velocity sufficient to form a fully expanded bed in the purge vessel. Although this process overcomes many disadvantages incident to prior art techniques, it is not the simplest process, since the process requires the use of a gas permeable, solids impermeable constraint means to confine solids and suppress bubbling fluidization.
Accordingly, there is a need for a more effective process and apparatus for purging solid olefin polymers. Surprisingly, the present invention is an improvement over existing systems because it affords greater latitude in adjusting or regulating the superficial gas velocity to result in more efficient purging in less time without the use of a gas permeable, solids impermeable constraint means. The process and apparatus of the present invention more effectively provides that individual solid particles in the bed of the purge vessel will be swept by the purge gas and thus offer improved solid-gas contact.